There is a long-standing need for valve devices which can be used to produce a variable fluid flow. Varying the flow through valves is usually achieved by varying the available cross-sectional area presented to the flow. Devices utilizing a disk occluder disposed in a fluid flow passageway, which co-operates with electromagnetic coil in order to open or close the passageway permitting or restricting fluid flow through the passageway are known in the art. A typical example of this type of valve is disclosed in U.S. Pat. No. 5,135,538, an electromagnetic control mechanism forces the disk occluder to remain closed when energised until a predetermined net pressure is reached against the disk occluder. Where the electromagnetic control mechanism is either selectively deenergized or the force on the occluder disk is greater than the holding force of the electromagnetic control mechanism, the disk occluder is released from the closed position to pivot on the fulcrum in response to fluid pressure.
A difficulty with this device resides in the difficulty of accurately controlling the position of the occluder disk, and achieving a graduated flow variation profile, particularly at the low flow rate end of the control and also the fact that the valve is opened when deenergized.
An example of another device, wherein a size of the cross-sectional area exposed to fluid flow is subject to magnetic flux, is disclosed in U.S. Pat. No. 3,970,112. The power to a coil is altered in order to alter an accumulation of small iron particles which are mixed with a controlled fluid to the inner wall of tubular member in a location near the spaced end surfaces and thereby restricts or allows fluid flow through the tubular member.
A main disadvantage of this device is a constant presence of iron particles in a controlled fluid which makes its use possible only in close-circuit systems and with fluids of particular types of chemical and physical properties.